1. Field of the Invention
This invention relates to a rubber composition comprising a specific high molecular weight styrene-diene copolymer rubber and a low molecular weight polymer and being flexible and having high tensile strength, hysteresis loss (hereinafter abbreviated as high-loss) and wear resistance.
2. Related Art Statement
The styrene-butadiene copolymer rubber (SBR) is produced in a greater amount as a so-called general-purpose rubber and has excellent in the quality and cost stabilization and also excellent antioxidation property and wear resistance as compared with the natural rubber, so that it is used in a greater amount as a starting material for automobile tires, particularly radial tire for automobile. Further, SBR is frequently used in a tread portion as a part of the radial tire for passenger car.
In the automobile tires, particularly radial tires for passenger car, the cornering stability is an important performance required in the tread portion. In order to ensure the cornering stability, it is necessary that the road gripping force (gripping property) of the tread as a high loss property is high. For this purpose, it is naturally attempted to improve the tread portion. As such a method, the improvement of tread pattern is attempted. In this method, by making deep the grooves for the formation of the pattern formed in the tread, the tread portion directly contacting with ground is easy to be deformed by the pressure applied from the road surface, whereby the ability of changing the pressure from the road surface into heat energy inside the tread, i.e. the ability of absorbing the pressure from the road surface to a certain extent is increased to improve the road gripping property of the tread.
However, in this method, the deformation of rubber is made easy, so that the worn amount of the tread portion increases and the sufficient wear resistance can not be obtained.
In order to increase the wear resistance of rubber as a tread material, a method of increasing the amount of high reinforcing carbon black filled in rubber is performed from the old times. According to this method, however, the hardness and modulus rise and it is difficult to deform the tread portion, and consequently the road gripping property is undesirably damaged.
Further, there is a method of improving the wear resistance by increasing the Mooney viscosity (a parameter of average molecular weight) of SBR as a tread material. In this method, however, the processability of SBR unfavorably lowers, so that the method itself is critical.
Moreover, there is generally proposed a method of improving the reduction of heat generation in the working and the resistance to uneven wear by combining a high molecular weight polymer with a liquid polymer. However, this is still insufficient as a tread material from a viewpoint of the balance between the gripping property and the wear resistance.
In addition, it is attempted to simultaneously establish the gripping property and the wear resistance by blending with a rubber component having a high glass transition temperature. However, the wear resistance is hardly improved by this method.
As mentioned above, the road gripping property and the wear resistance in the tire tread have a so-called conflicting relation even when using the known rubber materials, so that the improvement of both properties is considered to be very difficult.